Raising drives are often used with driven machines and serve the purpose of fixing equipment. In connection with agricultural vehicles, for example tractors, the raising drives are also called tool bars. In agricultural equipment, for example, a plough, can be fixed and can be moved upwards and downwards by means of the raising drive. Also the vertical position of a piece of equipment is determined by the raising drive.
Usually, raising drives have at least one single-actuated hydraulic cylinder, which is connected to a control arrangement, the control depending on various measuring signals and instructions given by operators.
For example, a piece of equipment fixed on the raising drive is raised by means of a pressure activation of the hydraulic cylinder. The lowering then takes place by means of pressure relief of the hydraulic cylinder due to the effect of the gravity, the lowering speed depending on, for example, the mass of the equipment and frictional forces, if any. In order to limit the lowering speed, fixed or adjustable valves or throttles are inserted in a working pipe of the hydraulic cylinder, fluid flowing through said valves or throttles during the lowering.
Usually, it is desirable to maintain the position of the equipment. For example, with a soil treatment tool, a penetration depth of the tool specified by the driver is considered by the control. However, observing the prespecified penetration depth of the tool should not cause the tractor to bog down.
U.S. Pat. No. 4,518,044, for example, discloses a control, which, apart from the penetration depth of the tool specified by the driver, also considers the slip of the driving wheels, the speed of the motor, and the drag of the tool during the control. In this connection, the raising and lowering speeds are controlled in that with corresponding speed various desired positions are specified. The actual raising and lowering speeds are not measured in this connection.
This may particularly cause the lowering speed to deviate from the desired speed. With a light-weight tool, frictional forces may prevent the tool from following the prespecified desired positions during lowering, for example, the tool moves slower than anticipated. With a heavy-weight tool it may occur that the actual position is ahead of the desired position, meaning that the tool moves faster than intended.
With soil treatment tools there is an additional problem, namely that the effect of the gravity is substantially eliminated, when the tool gets in contact with the surface of the ground. Particularly with heavy-weight tools, the user will normally have partially closed the throttle or the valve in the working pipe, in order to limit the lowering speed. With a slow lowering speed and/or with light-weight tools it may, however, happen that the gravity is no longer sufficient to lower the tool into the ground against the frictional forces of the system and the forces of the soil acting against the gravity.
It is known for the user to solve this problem by increasing the lowering speed, that is, to further open the throttle or the valve in the working pipe, as soon as the tool hits the ground. This causes a further relief of the hydraulic cylinder, which again reduces the forces acting against the penetration of the tool into the ground.
It is also known to provide the control with a so-called fast penetration function, which makes it possible for the driver to activate a switch at the contact of the tool with the ground, after which the control, for example, opens a valve aperture to its maximum to completely relieve the hydraulic cylinder. Such solutions are, however, less comfortable, and may under certain circumstances distract the driver.
In order to achieve the same lowering speeds in connection with tools having different masses or weights, for example, U.S. Pat. No. 5,684,691 discloses the measuring of a parameter, which is proportional to the weight of the tool. In this connection, the weight of the tool is measured by weighing with still-standing vehicle and raised tool. Depending on the measured weight, the tools are then divided into categories. The specifications of desired raising and lowering speeds are then made in accordance with the individual categories. This solution has the disadvantages that the lowering speed is substantially reduced at the contact of the tool with the ground and that each change of tool requires a weighing process.
It is also known, for example from U.S. Pat. No. 4,571,500 and EP 0 414 279 A2, to determine the lowering speed and adapt the control correspondingly. Thus, the lowering speed can be controlled independently of the weight of the tool. The actual lowering speed is determined by differentiation of position signals. In connection with faulty position signals, malfunction may easily occur, the differentiation increasing the influence of such malfunctions. As the control of the lowering speed takes place in parallel to the control of the position of the tool, instabilities, such as, for example, oscillations may occur.